Description
This post describes what Short Guard Interval is, how it is used on FortiAPs, and when to enable this feature.
Scope
FortiAP supporting 802.11n and 802.11ac.
Solution
A Guard Interval (GI) is a period of time between symbol transmissions. It allows reflections from previous data transmissions to settle before transmitting a new symbol. The signal content inside the GI, called Inter-Symbol Interference (ISI), is rejected by receivers. 802.11a/g OFDM transceivers use 800 ns GIs.
The HT PHY introduced two guard intervals: 400ns (Short Guard Interval SGI) and 800 ns (Long Guard Interval LGI). Support of the 400ns SGI is optional for transmitters and receivers. The VHT PHY also supports SGI and LGI of the same timing. Figure 7.4 shows the difference between these two.
Choosing a short guard interval on an environment with multipath leads to more ISI and lower throughput. A long guard interval means more idle time on the wireless medium, which is wasteful on some other environments. A good Guard Interval helps to get the most out of throughput between AP and STA. The selection of the guard interval type depends on various factors such as the specific WiFi standard being used (e.g., 802.11n, 802.11ac), the characteristics of the wireless environment, and the desired balance between data throughput and reliability. With SGI enabled in highly reflective environments, users may experience higher packet error rates; using LGI instead is recommended in these cases. It is recommended to talk with Fortinet's Professional Services to get advice on whether or not this feature should be enabled or not on a specific environment.
A 400ns GI adds about 9-10% to the data rate over an 800ns GI. This is shown in the 802.11n (HT) and 802.11ac (VHT) MCS tables. See https://www.cwnp.com/802-11n-guard-intervals-gi/ for a detailed explanation of SGI and https://wlanprofessionals.com/mcs-table-and-how-to-use-it/ for a detail explanation on the MCS table.
By default Long Guard Interval (800 ns GI) is used on FortiAP Profiles. Short Guard Interval (400 ns GI) can be enabled on the FortiAP profile through CLI or GUI.
config wireless-controller wtp-profile
edit "<profile-name>"
config radio-1
set short-guard-interval [enable|disable]
end
config radio-2
set short-guard-interval [enable|disable]
end
config radio-3
set short-guard-interval [enable|disable]
end
config radio-4
set short-guard-interval [enable|disable]
end
next
end
Example:
On a Lab environment, Short Guard Interval was enabled on the WTP profile of a FortiAP 221E. See below what is observed:
